Automatic frequency control circuit for frequency modulation television systems



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o W0 40 1x 120 l k m W. E. BRADLEY AUTOMATIC FREQUENCY CONTROL CIRCUITFOR FREQUENCY MODULATION TELEVISION SYSTEM Flled Aprll 10 1946 Sept. 13,1949:

Patented Sept. 13,1949

AUTOMATIC FREQUENCY CONTROL CIR- CUIT FOR FREQUENCY MODULATIONTELEVISION SYSTEMS William E. Bradley, Swarthmore, Pa., assignor, bymesne assignments, to Philco Corporation, Philadelphia, Pa., acorporation of Pennsylvania Application April 10, 1946, Serial'No.660,866

9 Claims. 1

This invention relates to frequency-modulation carrier wave systems, andmore particularly to unsymmetrlcally modulated frequency-modulationsystems wherein the modulated carrier signal includes differentmodulation components, one of which varies over a predeterminedfrequency deviation range of the carrier signal while another variesover a deviation range beyond the range of variation of the firstcomponent. A specific and important example of such a system is afrequency-modulation television system wherein the carrier wave, duringthe video intervals; varies within certain predetermined fixed frequencylimits, while the carrier wave, during the synchronizing intervals, isshifted to a preassigned frequency beyond the range sf vari ationobtainin during the video intervals. For proper operation, it isessential that either the black level (i. e. the blanking signal level)or the synchronizing signal level of the modulated carrier be maintainedsubstantially constant at its preassigned frequency value in order topreserve the D. C. component of the video signal.

In some instances, it is desirable or expedient to convert the modulatedcarrier to a different frequency. For example, the modulated carrier maybe transmitted through one or more relay stations or transmitters, andthe latter ma convert the modulated carrier from one frequency toanother. This may be accomplished by a conventional frequency converterarrangement employing a beating oscillator. Such frequency conversion,however, may cause frequency drift of the converted signal due to theinherent tendency of the beating oscillator to drift or to have itsfrequency varied by some circuit condition.

The principal object of the present invention is to provide an automaticfrequency control system adapted to maintain an unsymmetricallymodulated frequency-modulated carrier wave within a preassigned wavechannel.

A more specific object of the invention is to provide, in afrequency-modulation television system, an arrangement by means of whichthe black-level frequency may be maintained at a predetermined fixedvalue.

' 2 bined action of two forces, as hereinafter described.

A further object of the invention is to provide a novel automaticfrequency control arrangement in a frequency-modulation television sys-.

trations which will facilitate a clear understand Referring particularlyto Fig. 1, there is illustrated in block form an FM television relay system comprising an amplifier I, a mixer stage 2, a' second amplifier 3,and a beating oscillator These elements may be of conventional form, andtherefore, it is unnecessary to illustrate or describe them in detail.The amplifier 1 is adapted to receive the incoming frequency-modulatedcarrier signal, which is converted to a different frequency in the mixerstage 2 by the heterodyne action of the beating oscillator 4, as will'be well understood. The amplifier 3 is adapted to transmit theconverted modulated carrier, which may be transmitted for reception atthe new frequency or may be transmitted to another relay station ortransmitter. By way of example, it may be assumed that the modulatedcarrier received by the system in Fig. 1 has a band center frequency ofme. and a maximum frequency deviation (from center frequency) of 10 mc.In such case, the amplifier I should be adapted to operate at afrequency of 75 mc. and should have a pass band of 20 me. It may beassumed further that the incoming signal is to be converted from '75 me.to mc., and that the oscillator 4 is adapted to operate at a frequencyof mc. to give such frequency conversion. Accordingly the amplifier 3may be adapted to operate at a frequency of 115 mc. and may have a passband of 20 me.

' Fig. 2 illustrates the general character of the frequency-modulatedcarrier signal employed in an FM television system. In thisillustration, the ordinate line represents different frequency values.The dot-and-dash line represents the whitelevel frequency Jw of thevideo component of the modulated carrier; the dashed line represents thepreassigned peak frequency f5 of the synchronizing pulses P, and thedotted line represents the preassigned black or blanking level frequencyfb- The video component V of the modulated carrier varies between thefrequency limits in and fw. The synchronizing component comprises thetime-spaced pulses P, the peaks of which should be accurately levelledalong the line is. The purpose of the present invention is to effectautomatic frequency control in a system employing such a signal,particularly in a system of the character shown in Fig. 1.

Automatic frequency control (A. F. 0.) systems adapted accurately to fixthe mean frequency of frequency modulation transmitters of the type usedin sound broadcasting are, of course, well known in the art. These knownsystems employ circuits responsive to the mean transmission frequencyand are adapted to maintain the said mean frequency at a preassignedfrequency value. These prior devices, are, however, not adapted for usein conjunction with frequency modulated television transmitters orreceivers for the reason that there is no mean carrier frequency in anyvalid sense of the word. What would otherwise pass for the meanfrequency is, in a television system, a function of the video signal andvaries with the average tone of the picture being transmitted. Thus ifthe picture were very dark in tone the mean frequency would be very nearthe black level is, while if the picture were very light in tone themean frequency would be found very near the white level ,fw. Thus in afrequency modulated television system the term mean frequency ismeaningless and prior A. F. C. systems which rely for their operationupon the existence of a mean frequency are inapplicable.

In accordance with the present invention frequency control means areprovided which rely for their operation not upon the existence of a meanfrequency but rather upon the presence of a fixed, unvarying referencefrequency occurring cyclically at a relatively high frequency rate. Inpractice this reference frequency preferably comprises either theblanking level frequency is or the synchronizing peak frequency is.

As previously stated, when a frequency modulated television signal issubjected to a frequency conversion process in a system such asillustrated in Fig. 1, the inherent tendency of the beating oscillatorto drift may cause drift of the converted signal. In accordance with thepresent invention, this undesirable frequency drift of the convertedsignal is prevented by means of the arrangement now to be described. Thearrangement will also, of course, compensate for frequency driftoccurring at an earlier point in the system.

A deviator 5 is provided in conjunction with the oscillator 4 to varythe frequency thereof in. re-

sponse to a control voltage supplied by way ofconnection 6. The deviator5 maytake the form of any conventional device which is adapted to varythe frequency of an oscillator in response to a control voltage. Suchdevices are well known, particularly in automatic frequency controlarrangements. In accordance withthe present invention the deviator 5 isactuated by a control voltage supplied by the frequency-selectivedetector, or discriminator, arrangement designated as a. whole by thedotted line rectangle 1.

. diode load resistors.

Referring specifically to the detector arrangement, there is provided aresonant circuit 8, preferably of relatively low Q, tuned approximatelyto a frequency midway between the frequencies is and fw of the convertedcarrier signal. The circuit 8 is arranged toreceive a portion of thesignal energy from the amplifier 3 to which it may be coupled in anysuitable manner. For example, the said circuit may be connected to theanode of the last tub'e, represented at I2, through a suitable couplingcondenser I3, the load impedance of the tube being represented at M. Theresonant circuit 8 comprises a capacitance element 9, an inductanceelement l0, and a resistance element ll, all connected in shuntrelation.

It will be understood that the resistance element I imparts the desiredlow Q characteristic to the circuit. A diode detector |5 has its cathodeconnected to the high potential side of the resonant circuit 8, whilethe diode anode is connected to one side of the resistance-capacitanceload circuit [6. The latter is connected to the low potential side ofresonant circuit 8 through the indicated ground connections.

A second resonant circuit preferably of relatively high Q, is alsoarranged to receieve a portion of the signal energy. This isconveniently accomplished by loose reactive coupling with the low Qresonant circuit 8, as indicated by the bracket and letter M. Theresonant circuit comprises shunt-connected capacitance and inductanceelements It and I9, respectively, and this circuit is tuned to afrequency just beyond the peak frequency of the synchronizing pulses,the purpose of which will appear presently. A second diode detector 20has its anode connected to the high potential side of the resonantcircuit l1, while the diode cathode is connected to aresistance-capacitance load circuit 2| which, in turn, is connected tothe low potential side of the resonant circuit I! through theillustrated ground connections.

The time constants of circuits l6 and 2| are both large in comparison tothe time interval between the synchronizing pulses, and unidirectionalvoltages of negative and positive polarity, respectively, areestablished thereacross. When the system is properly balanced, thesevoltages tend to be of the same order of magnitude, one of the voltagesnormally predominating slightly.

The opposite-polarity voltages established across circuits l6 and 2|are-applied to conductor 6 by way of resistors 22 and 23, which may havevalues substantially equal to or larger. than the The resistors 22 and23 form a high resistance voltagedivider extending between the outputterminals of the diode load circuits I6 and 2|. Since this voltagedivider is center-tapped by the conductor 6, it will be apparent that anaverage of the output voltages of the two detectors is derived by way ofconductor 6. Thus if the voltages were equal, the average voltage onconductor 6 would be zero. In a welldesigned system, the voltage onconductor 6 will not vary widely from such a balanced condition.

The operation of the system may be clearly understood with the aid ofthe illustration of Fig. 3. In that figure, there is shown the output orresponse characteristic of the frequency selective detectorarrangement 1. Portion 24 of theillustrated curve represents theresponse characteristic of the low Q- circuit 8 and itsassociateddetector, while the sharply peaked portion" 25 represents theresponse characteristic of thehigh Q circuit IT and its associateddetector.

As previously mentioned, the low Q circuit 8 is tuned to a frequencyintermediate the frequencies flu and fw, while the high Q circuit I1 istuned to a frequency ,fx just beyond the peak frequency is of thesynchronizing pulses. To further facilitate an understanding of theoperation, a portion of the modulated carrier signal is represented inassociation with the response curve.

In operation, assume that the negative voltage across the detector loadcircuit I6 is slightly greater than the positive voltage across circuit2|, so that a small negative voltage is applied to the deviator '5. Thissmall votlage tendsto change the frequency of the oscillator 4 by asmall amount in a direction to shift the synchronizing pulses 1? towardthe frequency value is. The force exerted by the small bias voltage maybe likened to a nudging action. As a result of such action, thesynchronizing pulse tips enter the region of substantial response of thehigh Q circuit I1.

This results in an increased positive output from the detector system2ll-2l and a consequent decrease in the negative voltage applied to thedeviator. Thus the action of the arrangement provided by the inventioninvolves two opposing forces, one of which acts to nudge thesynchronizing pulses in a direction to bring the other force intoplay.By the conjoint action of the two forces, the preassigned frequencylimits in and jw of the video carrier signal are prevented from shiftingto any appreciable extent, by effectively leveling the frequencymodulated composite signal along the peaks of the synchronizing pulses.The very steep side slope of the response characteristic of the high Qcircuit I! greatly enhances this leveling action.

In the design of the discriminator I, it is important that while the Qof the resonant circuit I! should be high compared to the Q of resonantcircuit 8, it must not be so high as to result in unduly sustainedringing when shock excited @y the synchronizing signal or by sidebandsof the video components of the carrier signal. In general the Q of thecircuit I! is preferably held within such limits as will ensure aringing duration of no more than a fraction (e. g. A; or less) of thehorizontal synchronizing signal duration. In other words, the A.-C. timeconstant of the high Q circuit should be short compared to the durationof the synchronizing pulse.

The use of a differentially-connected frequency discriminator in thepresent invention is of great practical importance, in that through itsuse there is provided. in the A. F. C. system, a fixed referencefrequency or zero crossover point (see Fig. 3) at which thediscriminator output is substantially independent of the amplitude ofthe signal applied to the discriminator circuit 1. By so adjusting thediscriminator components that this crossover point occurs near, andpreferably between, the frequencies is and fb great accuracy offrequency levelling is provided, independently of signal strength.

In the system illustrated, only one frequency conversion is involved,but it will be apparent that such a system may involve more than onefrequency conversion, and the invention may be utilized to preventfrequency drift at each point of conversion. It will also be apparentthat the invention is not limited to the specific apparatus shown but iscapable of various modifications within the scope of the appendedclaims.

I claim:

1. In a frequency modulation television sys- 6. tem, a carrier wavesource, the carrier wav'e supplied by'said source being frequencymodulated with both video and synchronizin intelligence, an oscillatorof controllable frequency, means responsive to frequency variationswithin the deviation limits of the video intelligence components of saidcarrier wave for producing a first control voltage, said responsivemeans having a substantially flat frequency-response characteristic atfrequencies within said video-intelligence deviation limits, meansresponsive to a frequency near the synchronizing-signal deviation limitsof said carrier wave for producing a second control voltage, saidlast-mentioned responsive means having a steep frequency-responsecharacteristic at frequencies near said synchronizing-signal deviationlimits, and means for utilizing said control voltages differentially tocontrol the frequency of said oscillator.

2. A frequency modulation television system as claimed in claim 1,characterized in that said first and second control voltage producingmeans, in combination, possess an output-versus-frequency characteristichaving a reference frequency, within the deviation limits of saidsynchronizing signal, at which the differentially-combined controlvoltage output is substantially zero irrespective of the amplitude ofsaid carrier Wave.

3. Ina frequency-modulation receiver; an automatic-frequency-controlsystem comprising a source of 'asymmetrically-frequency-modulatedcarrier signal including different modulation components, one of saidcomponents varying within a preassigned carrier frequency range andanother component extending outside said preassigned range; meansincluding a beating oscillator for converting the carrier signal to adifferent frequency band; means for varying the frequency of saidoscillator; means responsive to frequency variations of said first-namedcomponent of said converted carrier for so actuating saidfrequency-varying means as to urge the oscillator frequency in onedirection, said responsive means having a substantially fiatfrequencyresponse characteristic throughout the range of frequencies ofsaid first-named component as converted; and means responsive to saidother component of said converted carrier for so actuating saidfrequency-varying means as to urge the oscillator frequency in the otherdirection, said last-named responsive means having a steepfrequency-response characteristic at frequencies in the region of theconverted frequency of said other component.

4. In a frequency-modulation television system employing a carrier wavewhich is frequency modulated with signals including video signals andsynchronizing pulses, the wave frequency of said synchronizing pulsesbeing outside the wave band of said video signals, the location of saidmodulated wave in the frequency spectrum being a function of theoperating frequency of a controllable oscillator; means responsive tofrequency variations of the video component of the carrier wave forproducing a control voltage, said responsive means having asubstantially flat frequency response characteristic throughout therange of said video component of said carrier wave; means responsive toa frequency in the neighborhood of the frequency of thesynchronizingcomponent of the carrier wave for producing another controlvoltage, said last-named responsive means having a steepfrequency-response characteristic at frequencies in said neighborhood;and means for utilizing said control voltages differentially to controlthe frequency of saidoscillator so as to maintain the black-levelfrequency of the carrier wave substantially constant. l

5. In a frequency-modulation television system employing a carrier wavewhich is frequency modulated with'signals including video signals andsynchronizing pulses, the wave frequency of said synchronizing pulsesbeing outside the wave band of said video signals, the location of saidmodulated wave in the frequency spectrum being a function of theoperating frequency of a controllable oscillator; means responsive tofrequency variations of the video component of the carrier wave forproducing a control voltage, said responsive means having asubstantially flat frequency response characteristic throughout therange of said video component of said carrier wave; means responsive toa frequency just beyond the frequency of the synchronizing comfrequencyof the synchronizing component of the carrier wave; and means forutilizing both of said control voltages to control the frequency of saidoscillator so as to maintain the black-level frequency of the carrierwave substantially constant.

6. 'In a frequency-modulation television system employing a carrier wavewhich is frequency modulated with signals including video signals andsynchronizing pulses, the wave frequency of said synchronizing pulsesbeing outside the wave band. ofsaid video signals, the location of saidmodulated wave in the frequency spectrum being a function of theoperating frequency of a controllable oscillator; means for varying thefrequency of said oscillator; means responsive to the video componentsof the carrier wave for actuating said frequency-varying means so as tourge the oscillator frequency in one direction, said responsive meanshaving a substantially fiat frequency response to said video components;and means responsive to synchronizing components of the carrier wave foractuating said frequencyvarying means so as to urge the oscillatorfrequency in the other direction, said last-named responsive meanshaving a. steep frequency-response characteristic at frequencies near tosaid synchronizing components.

.7. In a frequency-modulation television sys tem employing a carrierWave which is frequency. modulated with signals including video signalsand synchronizing pulses, the Wave frequency of said synchronizingpulses being outside the wave band of said video signals, the locationof said modulated wave in the frequency spectrum being a function of theoperating frequency of a con trollable oscillator; means for varying thefrequency of said oscillator; a frequency-variation response networkhaving a substantially fiat frequency response to video frequencycomponents of the carrier wave; a detector coupled to said network andadapted to produce a unidirectional control Voltage of certain polarity;a frequencyvariation response network having a steep frequency-responsecharacteristic at frequencies just beyond the frequency of thesynchronizing component of the carrier wave, said last-mentioned networkbeing reactively coupled to said first-mentioned network; a detectorcoupled to said last-mentioned network and adapted to produceaunidirectional control voltage having .a'

polarity opposite that of the first-mentioned control voltage; and meansfor applying said control voltages to said frequency-varying means,the'polarity of said first-mentioned control voltage being suchas tourge the frequency of the synchronizing component toward the frequencyto which said last-mentioned network is most responsive, while thesecond control voltage acts in opposition to such change.

8. In a frequency-modulation television system employing a carrier Wavewhich is frequency modulated with signals including video signals andsynchronizing pulses, the wave frequency of said synchronizing pulsesbeing outside the wave band 1' of said video signals, the location ofsaid modulated wave in the frequency spectrum being a function of theoperating frequency of a controllable oscillator; means for varying thefrequency of said oscillator; a frequency-variation response networkhaving a substantially flat frequency response to Video components ofthe carrier wave and arranged to receive a portion of the signal energy;a detector coupled to said network and adapted to produce aunidirectional control voltage of certain polarity; afrequency-variation response network having a steep frequencyresponsecharacteristic at frequencies just beyond the peak frequency of thesynchronizing pulses of the carrier wave, said last-mentioned networkalso being arranged to receive a portion of the signal energy; adetector coupled to said last-mentioned network and adapted to produce aunidirectional control voltage having a polarity opposite that of saidfirst-mentioned control voltage; and means for applying said controlvoltages to said frequency-varying means, the polarity of saidfirst-mentioned control voltage being such as to urge the peaks of saidsynchronizin pulses toward the frequency to which said last-mentionednetwork is most responsive, while the second control voltage acts toprevent any substantial shift of said peaks.

9. In a frequency-modulation television system employing a carrier wavewhich is frequency modulated with signals including video signals andsynchronizingpulses, the wave frequency of said synchronizing pulsesbeing outside the wave band of said video signals, the location of saidmodulated wave in the frequency spectrum being a function of theOperating frequency of a controllable oscillator; a frequencydiscriminator responsive to at least a portion of th frequency range ofsaid frequency-modulated carrier wave, said discriminator comprisingelements including a frequency variation response network having asubstantially fiat frequency response to frequencies within thevideo-signal range of said Wave, and a frequency-variation responsenetwork having a steep frequency-response characteristic at frequencieswithin the synchronizingpulse range of said wave, the time constant ofsaid last-mentioned network bein small compared to the duration of asynchronizing pulse, the output-versus-frequency characteristic of saiddiscriminator having a zero-crossover point in the range of frequenciesoccupied by said synchronizing pulses; and means responsive to an outputsignal of said frequenc discriminator for controlling the frequency ofsaid oscillator to maintain the synchronizing-level frequency of saidwave substantially constant.

WILLIAM E. BRADLEY,

(References on following page)- REFERENCES CITED Number Name Date Thefollowing references are of record in the g ig g is file of thisparbent: Q mson 2,290,517 W11son July 21, 1942 UNITED STATES PATENTS 52,296,919 Goldstine Sept. 29, 1942 2 341 649 Peterson Feb. 15 1944Number Name Date 2 44 2 5 Travis Jan 17 19 9 2:354827 Peterson Aug. 19442,413,913 Duke Jan. 7, 1947 Disclaimer 2,481,9O2.-Wz'll2'am E. Bmdley,Swarthmore, Pa. AUTOMATIC FREQUENCY CONTROL CIRCUIT FOR FREQUENCYMODULATION TELEVISION SYSTEMS. Patent dated Sept. 13, 1949. Disclaimerfiled Jan. 17, 1952, by the assignee, Phz'lco Corporation. Hereby entersthis disclaimer to claims 1, 3, 4, 5, 6, and 8, of said patent.

[Oyficial Gazette February 19, 1952.]

